3. Nature's Wildest Weapons: Horns, Tusks and Antlers

The animal kingdom possesses a fearsome arsenal - a variety of extreme weapons used in epic battles to gore, stab, crush and batter. But are they more than just instruments of deadly force? For Professor Doug Emlen, it has been his lifetime's ambition to discover the secrets of nature's arms races, what triggers them and what they can teach us about the most formidable weapons on earth - our own.